Rachel Morello-Frosch, professor Department of Environmental Science, Policy & Management & School of Public Health is seen at Giannini Hall at UC Berkeley on Tuesday, February 5, 2013 in Berkeley, Calif.

Mothers who breathe the kind of pollution emitted by vehicles, coal power plants and factories are significantly likelier to give birth to underweight children than mothers living in less polluted areas, according to international findings published Wednesday.

The study is believed to be the largest to examine how newborns' bodies are affected by air quality, an issue that has raised particular concern in China and other developing nations.

Nearly 30 researchers, including three from the Bay Area, based their conclusions on more than 3 million births at 14 sites in North America, South America, Europe, Asia and Australia.

Focusing on children born on-time in the mid-1990s to mid-2000s, they found that, worldwide, the greater the air pollution, the less babies tend to weigh at birth.

Chronic health issues

Weighing less than 5.5 pounds at birth is a factor for chronic health issues in childhood, including a higher risk for infection and developmental delays, experts say. Problems in adulthood can include cardiovascular disease, diabetes and other metabolic disorders.

"Being low birth weight basically is like you're starting at a little bit of disadvantage in terms of health throughout your lifetime," said Tracey Woodruff, the study's co-principal investigator and director of UCSF's Program on Reproductive Health and the Environment.

For the study, which appears in the journal Environmental Health Perspectives, researchers looked at two types of air pollutants, including inhalable coarse particles, which are about 10 micrometers in diameter and often appear in natural elements such as dirt, dust and sea salt.

Levels from 14 sites

The particles were found in various levels throughout the 14 sites. Seoul's air had the highest concentration, 66.5 micrograms per cubic meter, while Vancouver's had the lowest, 12.5 micrograms per cubic meter.

In the United States, California's levels - about 29 micrograms per cubic meter - exceeded those of Connecticut, Massachusetts, New Jersey and Atlanta. But the Golden State fared better than urban regions in Brazil, Italy and the Netherlands, where concentrations were in the 40s.

"It's all relative in terms of how you decide how bad California is," said Rachel Morello-Frosch, a UC Berkeley professor of environmental science, policy and management and public health. She and UC Berkeley researcher Bill Jesdale analyzed 1.7 million California births for the study.

The research showed that infants' risk of having a low birth weight rose by 3 percent with every increase of 10 micrograms per cubic meter in inhalable coarse particles. Overall, with each increase, infants were born 3 grams lighter.

When the study factored in individual variables, such as the mother's age and tobacco use, the average weight drop tripled to 9 grams.

Fine particles' strong effect

The effect appeared to be even more dramatic with another type of air pollutant, fine particles. These are 2.5 micrometers in diameter and can come from forest fires, power plants, factories and cars.

For each increase in fine particles by 10 micrograms per cubic meter, there was a 10 percent higher chance that newborns had a low birth weight when individual variables were taken into account, the researchers said.

Air pollution may have a small impact on an individual mother and her child. But in a large population, it could lead to a significant increase in the number of low-birth-weight babies, Morello-Frosch said.

In the United States, the yearly average concentration of fine particles in the air must be no more than 12 micrograms per cubic meter. In contrast, the European Union's limit is twice as high - 25 micrograms per cubic meter - and regulatory agencies are considering lowering it.

Push for regulation

The onus is on policymakers, not on mothers, to improve conditions, researchers said.

"This really speaks to the need for regulatory action to ensure that air pollution levels are consistently regulated at levels that protect public health and, in particular, protect prenatal and perinatal health," Morello-Frosch said.

The study's message resonates in light of the air-quality crisis in Beijing, where the density of fine particles has reached extremely hazardous levels, said Beate Ritz, a UCLA epidemiologist who has studied birth outcomes and air pollution in California. She was not part of the new study.

Ritz said the researchers have shown on a large scale what has until now been seen through smaller lenses: Air pollution can hurt future generations.

"Whatever impacts fetal growth and fetal development," she said, "we should really be worried about it."